Agree with what you said. The arrival of Casemiro might increase the strength of MU but that doesn't mean it can immediately make their gameplay better than before because indeed this cannot be determined by just one player entering because when it comes to gameplay, it is quite clear that this is related to the club's scheme and playing strategy and one player comes will not be able to change that if the other 10 are still playing monotonous.
Changes to Manchester United's gameplay will not happen immediately because players like Casemiro who came from the Spanish League to the English League also need to adapt himself to the style of play in the Manchester United team. And the Manchester United coach also has to change his strategy a bit to get a good form at the start of this season because Manchester United have lost twice in the previous game.